1. Field of the Invention
This invention relates to a CATV (cable television) system for delivering an FM stereo broadcast using frequency bands other than video broadcast frequency bands.
2. Background Art
When a stereo broadcast is delivered over a CATV system, ordinary FM broadcast receivers in households may be utilized to receive the stereo broadcast.
By this is meant that an audio stereo broadcast is transmitted using a wide frequency band ranging from 87.5 MHz to 120.0 MHz. In this case, television receivers for receiving images and FM tuners for receiving sound are provided in the subscribing households.
FIG. 1 shows a transmitter of the sort at the CATV center. In FIG. 1, there is shown an arrangement of a video/audio source 1, a TV/IF modulator 2, an IF scramble encoder 3 and a channel up converter 4 for converting the output into each channel frequency.
On the other hand, stereo sound for stereo broadcasting is supplied to a amplifier 7 through a stereo encoder 5 and an FM audio signal modulator 6. Subsequently, the images, monaural and stereo sounds are synthesized before being transmitted together with other channel signals.
FIG. 2 shows a receiver for receiving a stereo broadcast of this sort. The signal transmitted from the CATV center is supplied through a signal divider to a converter 14 and decoded by a scramble decoder 15 according to the data from a controller 16, the decoded signal being applied to a TV set 9.
On the other hand, the audio signal divided by the divider 35 and the outputs thereof are supplied by an FM stereo tuner 10 to speakers 12 and 13 through an audio amplifier 11.
In the FM stereo reception with the conventional CATV system, the video/audio source 1 is modulated by the TV/IF modulator 2 and supplied to the IF scramble encoder 3 as an IF video signal and an IF audio signal. The IF video signal is scrambled, whereas the IF audio signal is subjected to amplitude modulation and a descramble data code is superimposed thereon. This signal is frequency converted to a channel H by the channel up converter 4.
Apart from this, the audio stereo signal produced by the video/audio source 1 for stereo broadcasting is applied to the stereo encoder 5 and, after being subjected to frequency modulation in the FM audio signal modulator 6, is set by the multiple amplifier 7 to a frequency of, e.g., 84 MHz, for FM broadcasting.
The output signal thus produced is synthesized with the output signal of the above channel up converter 4 and transmitted together with other channel signals.
On the terminal side, a signal from a trunk line is divided by the divider 35 into those in the TV frequency band and those in the FM broadcast frequency band. The TV frequencies are supplied to a CATV terminal unit 8, whereas the FM frequencies are applied to an audio system consisting of the FM stereo tuner 10, the audio amplifier 11 and the speakers 12 and 13. When a viewer operates the terminal unit 8 with the intention of selecting the channel H (one of the stereo channels), the converter 14 tunes into the channel H under instructions from the controller 16 and the signal is supplied to the scramble decoder 15. Since the audio signal has been superimposed on the scramble decode data, the controller 16 provides the decode data according to the data demodulated from the audio signal, causing the descrambled TV signal to be produced and applied to the TV set 9. At this time, the viewer is expected to hear a monaural sound through the TV set 9.
Since the audio stereo signal on the FM broadcast band is transmitted by the CATV center, the view, if desirous of receiving the FM broadcast, manually operates the FM stereo tuner 10 located apart from the TV set 9, tunes in to 84 MHz corresponding to the channel H to receive FM stereo service by means of the FM stereo tuner.
As the conventional CATV stereo broadcasting is thus implemented, the viewer must mutually operate the FM stereo tuner with the TV set and is inconvenienced by operating the tuner whenever switching the TV channel.